1. Field of the Invention
The present invention relates to a moisture detector, a biological body moisture detector, a natural product moisture detector, and a product/material moisture detector. More specifically, the present invention relates to a moisture detector, a biological body moisture detector, a natural product moisture detector, and a product/material moisture detector that include a light-receiving element having a high sensitivity to a long-wavelength region of the near-infrared region without cooling.
2. Description of the Related Art
Moisture, biological components, medicines, environmental materials, and the like have an absorption band in the near-infrared region, and thus near-infrared spectroscopy has attracted attention as a noninvasive analytical method, and studies and practical applications thereof have been intensively performed. In addition, cosmic light reaching the ground during the day and night has a plurality of emission bands in the near-infrared region. Therefore, studies and practical applications of a method of capturing an image of light reflected from an object on the ground have been performed for commercial and military applications. In the analysis performed by near-infrared spectroscopy, output signals include necessary information and a large amount of noise due to characteristics of a light-receiving element. Accordingly, in order to extract necessary information from output signals without completely depending on an improvement in the performance of sensors (light-receiving elements), spectroscopic methods, chemometrics, and the like have been employed as important methods.
In the near-infrared region, the above sensors (light-receiving elements) are broadly divided into electron tubes and photodiodes (hereinafter also referred to as “PDs”) which are solid-state components. Among these sensors, research and development of PDs has been widely performed because PDs have a small size and can be easily highly integrated to form a one-dimensional array, a two-dimensional array, or the like. The present invention relates to a moisture detector including a PD. Currently, the following PDs or PD arrays are used. (1) An example such PDs or PD arrays is PDs or arrays thereof having a light-receiving sensitivity to the infrared region and also having a light-receiving sensitivity in the near-infrared region. Specific examples of such a photodiodes include a Ge-based PD, a PbS-based PD, an HgCdTe-based PD, one-dimensional arrays thereof, and two-dimensional arrays thereof. (2) Another example of such PDs or PD arrays is InP-based PDs, InGaAs-based PDs included in the category of the InP-based PDs, and arrays thereof. These photodiodes have a light-receiving sensitivity at a wavelength of 1.7 μm or less in the near-infrared region. Herein, the term “InP-based PDs” means PDs including an absorption layer made of a Group III-V compound semiconductor and provided on an InP substrate, and InGaAs-based PDs are also included in the InP-based PDs.
Among the above photodiodes, photodiodes described in (1) are often cooled in order to reduce noise. For example, most of the photodiodes are operated at the liquid nitrogen temperature (77 K) or using a Peltier device. Accordingly, commercial devices including such photodiodes have a large size, and thus the photodiodes described in (1) are not suitable for handy-type or shoulder-type portable near-infrared spectrometers, digital cameras, or the like. However, among such photodiodes, there is a sensor, e.g., a lead sulfide (PbS) sensor, that can be operated at room temperature. However, even when lead sulfide is used, the wavelength range in which light can be received is limited at room temperature. On the other hand, the InP-base PDs described in (2) have the following disadvantages: (I) In InGaAs, which is lattice-matched to InP, a dark current is low, but the light-receiving sensitivity is limited to a wavelength range of 1.7 μm of less in the near-infrared region. (II) In extended InGaAs wherein the wavelength region in which light can be received is extended to 2.6 μm, the dark current is high, and cooling is necessary. Accordingly, in the InP-based PDs, light having a wavelength at about 1.9 μm in a large absorption band of water cannot be used or it is necessary to cool the sensor in order to use the light.
Examples of light-receiving elements used in moisture detection to date will be described below. (C1) In a known method, among absorption bands of water, an absorption band of a wavelength in the range of 1.8 to 1.95 μm is used for measuring the moisture content by using PbS at room temperature (Japanese Unexamined Patent Application Publication Nos. 6-86902 and 2003-344279). (C2) A method of forming an image by light having a wavelength in the range of 1.2 to 2 μm with an infrared imaging detector has been disclosed (Japanese Unexamined Patent Application Publication Nos. 5-34281 and 2001-116689). Although the structure of the infrared imaging detector is not clearly described in this method, it is believed that a two-dimensional array of PbS-based PDs cooled using liquid nitrogen or a Peltier device is used. (C3) A measuring device including an InGaAs PIN photodiode has also been disclosed (Japanese Unexamined Patent Application Publication Nos. 8-86749 and 2007-10584). In addition, a method of detecting moisture using an InGaAs PIN photodiode, which is assumed from the wavelength region used, has been proposed (Japanese Unexamined Patent Application Publication No. 11-9897). (C4) The type of an element used as an infrared detector is not specifically described in some patent documents (Japanese Unexamined Patent Application Publication Nos. 7-260680, 7-159314, 2003-344278, and 10-118108). In these cases, considering the description that a name of “infrared sensor” or “infrared detector” is used for a light-receiving portion (i.e., an element that generates a voltage by receiving infrared rays) and from the wavelength used in the measurement, it is believed that lead sulfide described in (C1) above or a similar substance is probably used.
InGaAs PIN photodiodes have a problem that the light-receiving sensitivity must be extended to the long-wavelength side of the near-infrared region. However, this problem has not yet been satisfactorily solved in that noise and a dark current are suppressed while extending the light-receiving sensitivity to a wavelength of 1.7 μm or more (T. Murakami, H. Takahashi, M. Nakayama, Y. Miura, K. Takemoto, and D. Hara, “InxGa1-xAs/InAsyP1-y detector for near infrared (1 to 2.6 μm)”, Conference Proceedings of Indium Phosphide and Related Materials 1995, May, Sapporo, pp. 528-531 (Ref. 1), R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717 (Ref. 2), and Japanese Unexamined Patent Application Publication Nos. 9-219563 and 2001-144278).
As described in Japanese Unexamined Patent Application Publication Nos. 10-118108 and 9-219563 and Refs. 1 and 2, whose descriptions are omitted, there are some candidates for a light-receiving element and a light-receiving element array which does not require cooling with liquid nitrogen or a Peltier device and which has a light-receiving sensitivity at the long-wavelength side of the near-infrared region. However, there are many difficult problems to be overcome to realize practical use, for example, low crystallinity, a high dark current, and the difficulty of production, and thus such a light-receiving element and a light-receiving element array are still under development. Therefore, in a measurement using a photodiode not including a cooling mechanism, noise increases, and thus it is difficult to perform a measurement with a high accuracy. This problem becomes particularly serious in a detection of a very small amount of moisture.
In the above methods of analyzing moisture described in the above cited references, there is no reference to the sensitivity of a light-receiving element itself. All the references propose a preferable analytical method of moisture in accordance with properties of an object containing moisture. However, it is very significant that the performance of a light-receiving element itself is improved to realize a moisture detector suitable for a test object in accordance with the feature of the improvement in the performance. Specifically, if near-infrared spectroscopy can be performed using a photodiode in which a dark current is suppressed without using a cooling mechanism, useful information can be easily obtained with a high sensitivity. As a result, a development in many application fields of the photodiode can be promoted.